They'll do Shrek: The New Beginning. It will turn out not be the real Shrek at the end, though.

Naturally, fans of the series will sort of pretend like that movie was never made (unless pressured about it). Shrek 6 will feature Shrek himself returning to the big screen via a random and completely nonsensical occurence. Since it's impossible for this series to be any sillier about itself, it will become more serious as it goes on.